*Purpose: The purpose of the study was to describe the patterns of use and clinical/virologic outcomes of letermovir treatment for established CMV infection in transplant recipients.

*Methods: Multicenter observational study of SCT or SOT recipients treated for at least 7 days with letermovir for established CMV disease. Demographic, outcome, and safety data was collected.

*Results: 36 transplant recipients from 11 centers that had received LTV for established CMV infection were analyzed. The mean age was 57±12 years and 24 (65%) of patients were male. 15 patients (42%) were HSCT recipients and 22 (59%) were solid organ transplant (SOT) recipients; 1 patient had received both HSCT and SOT. 15 patients (42%) had proven/probable/possible end organ disease; 6 of those (40%) had resolution of disease prior to initiation of LTV.

26 patients (72%) had a viral load (VL) < 1000 IU/ml at the time of LTV initiation, typically as step down therapy due to intolerance/toxicity (n=16, 62%) or resistance (n=8, 31%). 9 of these had CMV end organ disease; 4 had clinically resolved at initiation. 25 patients were on LTV for more than 2 weeks and 23 had VL checked at week 2; 17 had an undetectable VL or below limit of quantification and 6 patients had a viral load of >137 IU/ml (2 >1000 IU/ml). Between weeks 5-7, 17 patients had VL checked; 13 had an undetectable VL and 4 had VL >137 IU/ml (1 >1000 IU/ml). 6 deaths occurred among patients with CMV VL <1000 at LTV initiation and none were attributed to CMV disease.

10 patients (28%) had a VL greater than 1000 IU/ml (range 1657-994,000, median 18270) at initiation of LTV with resistance the most common indication (n=7, 70%). 9 received LTV for >2 weeks and 8 patients had VL checked at week 2; 5 had a detectable VL of >137 IU/ml and 3 patients had VL >1000 IU/ml. 7 patients had VL checked between weeks 5-7 after starting LTV and 5 had detectable VL; 3 patients had VL >1000 IU/ml. 3 patients had initial decrease in VL after LTV started and later had rising VL while on therapy. 3 patients died, one directly attributed to CMV. Overall two patients stopped letermovir due to suspected adverse event.

*Conclusions: Patterns of LTV use for established CMV infection include as stepdown therapy after achieving low level viremia with relatively good short-term outcomes. More mixed outcomes were observed with higher VLs at initiation including increasing viral loads while on therapy. Letermovir overall was well tolerated. Future studies should address combination therapy and novel dosing strategies for refractory/resistant CMV or in those intolerant to available treatments.

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